Semiconductor-on-insulator (SOI) structures including gradient nitrided buried oxide (BOX)

ABSTRACT

A semiconductor-on-insulator structure includes a buried dielectric layer interposed between a base semiconductor substrate and a surface semiconductor layer. The buried dielectric layer comprises an oxide material that includes a nitrogen gradient that peaks at the interface of the buried dielectric layer with at least one of the base semiconductor substrate and surface semiconductor layer. The interface of the buried dielectric layer with the at least one of the base semiconductor substrate and surface semiconductor layer is abrupt, providing a transition in less than about 5 atomic layer thickness, and having less than about 10 angstroms RMS interfacial roughness. A second dielectric layer comprising an oxide dielectric material absent nitrogen may be located interposed between the buried dielectric layer and the surface semiconductor layer.

BACKGROUND

1. Field of the Invention

The invention relates to semiconductor devices, and more specifically tosemiconductor-on-insulator (SOI) structures and methods for fabricationthereof to improve metal oxide semiconductor field effect transistor(MOSFET) performance in complementary metal oxide semiconductor (CMOS)devices.

2. Description of the Related Art

The migration and segregation of mobile dopants, such as boron, to oxidematerials is a well known phenomenon in semiconductor structures. Forexample, the migration and segregation of boron to oxide isolationregions may cause narrow width effects in semiconductor channels whichin turn may impact a semiconductor device threshold voltage. Within thecontext of devices fabricated within semiconductor-on-insulator (SOI)structures, the impact of boron migration and segregation may be evenmore severe since there is typically also a buried oxide interfacelocated below a semiconductor device. In ultra-thin SOI (UTSOI) devices,the boron migration and segregation effect may become even more extremesince an oxide interface typically abuts a boron doped extension region.

A massive loss of boron into an underlying buried oxide (BOX) layer maylead to a high parasitic resistance and a compromised semiconductordevice performance. Loss of boron from a substrate into a buried oxide(BOX) layer can also affect operation of devices in which substratedoping is used as a back gate, or where substrate doping is used to seta threshold voltage as in the case of fully-depleted devices with a thinburied oxide (BOX) layer.

A potential solution to boron migration might be the use of a laser orflash anneal process for boron activation, but this solution severelyrestricts the process flow, and requires that a boron dopant in asemiconductor-on-insulator (SOI) structure never be exposed to aconventional spike anneal once the boron dopant is in place. Thissolution may lead to increased complexity in device integration, and toadded constrains in device design.

Novel buried oxide (BOX) layer structures that may be used withinsemiconductor-on-insulator (SOI) structures are disclosed in thesemiconductor fabrication art.

For example, Hsu, in U.S. Pat. No. 5,468,657, teaches a method fornitridation of a buried oxide (BOX) layer within asemiconductor-on-insulator (SOI) structure formed using a separation byimplantation of oxygen (SIMOX) method. The nitridation method usesnitrogen implantation to the same depth to which oxygen is implantedduring the separation by implantation of oxygen (SIMOX) method.

In general, one of the drawbacks of the SIMOX process is a highback-interface charge at the oxide/Si interface. The presence of backinterface charge can lead to mobility degradation (and hence reduceddrive current) in the channel of CMOS transistors fabricated on SIMOXsubstrates. The degradation is especially severe in CMOS devicesfabricated on SIMOX substrates where the active Si layer is thin (<20nm) as is the case for fully-depleted CMOS fabricated on ultra-thin SOI.

Semiconductor structures are certain to continue to increase in densityand decrease in dimension. As a result of these density and dimensionaltrends, desirable are additional semiconductor structures and methodsfor fabrication thereof that impede boron or other mobile dopantmigration and segregation to an oxide material, such as a buried oxide(BOX) material.

SUMMARY OF THE INVENTION

The invention includes semiconductor structures and methods forfabricating the semiconductor structures. The semiconductor structuresselectively incorporate nitrogen into a buried oxide (BOX) layer of asemiconductor-on-insulator (SOI) structure. The methods may be used totailor the location and concentration profile of the nitrogen in theburied oxide (BOX) layer to block the migration and segregation of boronor other mobile dopants from either an adjoining surface semiconductorlayer or an adjoining base semiconductor substrate into the buried oxide(BOX) layer.

A semiconductor-on-insulator (SOI) structure in accordance with theinvention includes a buried dielectric layer located interposed betweena base semiconductor substrate and a surface semiconductor layer. Theburied dielectric layer includes an oxide material having a nitrogengradient therein that increases at an interface of the buried dielectriclayer with at least one of the base semiconductor substrate and thesurface semiconductor layer. The buried dielectric layer has an abruptinterface with the at least one of the base semiconductor substrate andsurface semiconductor layer that transitions to the at least one of thebase semiconductor substrate and the surface semiconductor layer in lessthan about five atomic layers. Such an abrupt interface ischaracteristic of an interface formed by direct wafer bonding.

Another semiconductor-on-insulator (SOI) structure in accordance withthe invention also includes a buried dielectric layer located interposedbetween a base semiconductor substrate and a surface semiconductorlayer. The buried dielectric layer also includes an oxide materialhaving a nitrogen gradient therein that increases at an interface of theburied dielectric layer with at least one of the base semiconductorsubstrate and the surface semiconductor layer. The buried dielectriclayer has an interfacial roughness with the at least one of the basesemiconductor substrate and surface semiconductor layer of less thanabout 10 angstroms.RMS.

Yet another semiconductor-on-insulator (SOI) structure in accordancewith the invention also includes a buried dielectric layer locatedinterposed between a base semiconductor substrate and a surfacesemiconductor layer. The buried dielectric layer also includes an oxidematerial having a nitrogen gradient therein. Thesemiconductor-on-insulator (SOI) structure also includes a seconddielectric layer located interposed between the buried dielectric layerand the surface semiconductor layer. The second dielectric layercomprises an oxide dielectric material absent nitrogen.

A method for fabricating a semiconductor-on-insulator (SOI) structure inaccordance with the invention includes nitriding, while using a thermalnitriding method, an oxide layer located upon a base semiconductorsubstrate to provide a nitrided oxide layer having a first nitrogengradient that peaks at an interface of the nitrided oxide layer with thebase semiconductor substrate. The method also includes nitriding, whileusing a plasma nitriding method, the nitrided oxide layer to provide atwice nitrided oxide layer having a second nitrogen gradient that peaksat the exposed surface of the twice nitrided oxide layer. The methodalso includes laminating a surface semiconductor layer upon an exposedsurface of the twice nitrided oxide layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features, and advantages of the invention will become moreapparent upon consideration of the following detailed description, whichis read in conjunction with the accompanying drawings, wherein:

FIG. 1 shows a schematic cross-sectional diagram of a conventionalsemiconductor-on-insulator (SOI) structure, illustrating mobile dopant(e.g., boron) migration and segregation from a base semiconductorsubstrate and a surface semiconductor layer into a buried oxide (BOX)layer within the conventional semiconductor-on-insulator (SOI)structure.

FIG. 2 shows a schematic cross-sectional diagram of a pMOSFET fabricatedon a thin semiconductor-on-insulator (SOI) substrate illustrating mobiledopant (e.g., boron) migration and segregation from extension regionsinto a buried oxide (BOX) layer.

FIG. 3 shows a schematic cross-sectional diagram of an embodiment of asemiconductor-on-insulator (SOI) structure of the inventionincorporating tailored gradient nitrogen dopant concentrations atinterfaces of a buried oxide (BOX) layer with a base semiconductorsubstrate and a surface semiconductor layer.

FIGS. 4A-4F show a series of schematic cross-sectional diagramsillustrating the results of progressive stages in fabricating asemiconductor-on-insulator (SOI) structure in accordance with anembodiment of the invention.

FIGS. 5A and 5B show a pair of schematic cross-sectional diagramsillustrating the results of progressive stages in fabricating asemiconductor-on-insulator (SOI) structure in accordance with anadditional embodiment of the invention.

FIG. 6 and FIG. 7 show a pair of schematic cross-sectional diagrams of ametal oxide semiconductor field effect transistor (MOSFET) locatedwithin separate semiconductor-on-insulator (SOI) structures inaccordance with the embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention includes semiconductor-on-insulator (SOI) structures andmethods for fabricating the semiconductor-on-insulator (SOI) structures.The semiconductor-on-insulator (SOI) structures include nitrogenincorporated as a gradient within a buried oxide (BOX) layer. A nitrogengradient profile within the buried oxide (BOX) layer is optimized toinhibit mobile dopant (i.e., boron) migration and segregation from botha base semiconductor substrate and a surface semiconductor layeradjoining portions of the buried oxide (BOX) layer within thesemiconductor-on-insulator (SOI) structures.

The structures and methods of the embodiments are described in furtherdetail below, and understood within the context of the drawingsdescribed above. Since the drawings described above are intended forillustrative purposes only, the drawings are not necessarily drawn toscale.

FIG. 1 shows a schematic cross-sectional diagram of asemiconductor-on-insulator (SOI) structure in accordance with the priorart. The semiconductor-on-insulator (SOI) structure comprises a basesemiconductor substrate 30. A buried oxide (BOX) layer 20 is locatedupon the base semiconductor substrate 30. A surface semiconductor layer10 is located upon the buried oxide (BOX) layer 20.

FIG. 1 also shows mobile dopant (e.g., boron) atoms 40 thatpreferentially migrate and segregate into the buried oxide (BOX) layer20 from the base semiconductor substrate 30 and the surfacesemiconductor layer 10 during a generally high temperature (i.e., fromabout 800° to about 1200° C.) processing step.

FIG. 2 shows a schematic cross-sectional diagram of a MOSFET, i.e.,transistor T, (generally a p-MOSFET) fabricated on a comparatively thinsemiconductor-on-insulator (SOI) structure (i.e., the surfacesemiconductor layer 10 has a thickness from about 30 to about 350angstroms). The transistor T comprises a gate dielectric 51 located uponthe surface semiconductor layer 10. A gate electrode 52 is locatedaligned upon the gate dielectric 51, although such alignment is not alimitation of the prior art. A pair of spacers 53 (illustrated incross-section, but intended as representative of a single spacersurrounding the gate 52 in plan-view) adjoin opposite sidewalls of thegate electrode 52 and the gate dielectric 51. Finally, a pair ofsource/drain regions 54 is located within the surface semiconductorlayer 10 and separated by a channel region aligned beneath the gateelectrode 52.

FIG. 2 also illustrates migration and segregation of the mobile dopantatoms 40 into the buried oxide (BOX) layer 20 from: (1) a pair ofextension region portions 55 of the source/drain regions 54 (i.e.,portions of the source/drain region 54 beneath the spacers 53); and (2)the base semiconductor substrate 30. The mobile dopant atoms 40migration and segregation from the extension region portions 55 of thesource/drain region 54 into the buried oxide (BOX) layer 20 isexacerbated in a comparatively thin surface semiconductor layer 10(i.e., surface semiconductor layer thickness less than about 350angstroms) when the extension region portions 55 of the source/drainregions 54 abut the buried oxide (BOX) layer 20.

FIG. 3 shows a schematic cross-sectional diagram of an enhancedsemiconductor-on-insulator (SOI) structure in accordance with theinvention. The enhanced semiconductor-on-insulator (SOI) structureincludes a nitrogen gradient within the buried oxide (BOX) layer 20. Thenitrogen gradient comprises: (1) a first region of high nitrogenconcentration 60 within the buried oxide (BOX) layer 20 adjoining aninterface of the semiconductor surface layer 10 and the buried oxide(BOX) layer 20; and (2) a second region of high nitrogen concentration70 within the buried oxide (BOX) layer adjoining an interface of thebase semiconductor substrate 30 and the buried oxide (BOX) layer 20.

Each of the first region of high nitrogen concentration 60 and thesecond region of high nitrogen concentration 70 comprises a nitrogenconcentration from about 0.1 to about 10 atomic percent with respect toan aggregate of silicon atomic concentration and oxygen atomicconcentration (i.e., a silicon:oxygen:nitrogen atomic ratio for thefirst region of high nitrogen concentration 60 and the second region ofhigh nitrogen concentration 70 is from about 30:60:10 to about333:666:1. A region of the buried oxide (BOX) layer 20 interposedbetween the first region of high nitrogen concentration of 60 and thesecond region of high nitrogen concentration 70 has a nitrogenconcentration less than about 0.1 atomic percent. The nitrogen gradientis typically continuous, with maximum nitrogen concentrations within theburied oxide (BOX) layer 20 at the interfaces thereof with the surfacesemiconductor layer 10 and the base semiconductor substrate 30.

The embodiments that follow describe methods for fabricating asemiconductor-on-insulator (SOI) structure generally in accordance withthe semiconductor-on-insulator (SOI) structure whose schematiccross-sectional diagram is illustrated in FIG. 3.

FIG. 4A to FIG. 4F show a series of schematic cross-sectional diagramsillustrating the results of progressive stages in fabricating asemiconductor-on-insulator (SOI) structure in accordance with anembodiment of the invention. This embodiment of the invention comprisesa first embodiment of the invention.

FIG. 4A shows a semiconductor substrate 80. A buried oxide (BOX) layer90 is located upon the semiconductor substrate 80.

The semiconductor substrate 80 may comprise any of several semiconductormaterials. Non-limiting examples include silicon, germanium,silicon-germanium alloy, silicon carbide, silicon-germanium carbidealloy and compound semiconductor materials. Non-limiting examples ofcompound semiconductor materials include gallium arsenide, indiumarsenide and indium phosphide semiconductor materials. Typically, thesemiconductor substrate 80 comprises a silicon or silicon-germaniumalloy semiconductor material that has a thickness from about 1 to about3 mm.

The buried oxide (BOX) layer 90 comprises an oxide dielectric material.The oxide dielectric material may comprise an oxide of an element suchas silicon or germanium. Silicon oxide is a common oxide material. Otheroxide materials are not excluded. The oxide dielectric material may beformed using any of several methods. Layer transfer methods andlaminating methods are common. Also contemplated are deposition methods,such as chemical vapor deposition methods and physical vapor depositionmethods. Neither the instant embodiment, nor the invention, uses aseparation by implantation of oxygen (SIMOX) method for forming theburied oxide (BOX) layer 90, since an interface of a semiconductorsubstrate and a buried oxide (BOX) layer formed using a separation byimplantation of oxygen (SIMOX) method is often imprecisely defined andhas a relatively high concentration of fixed charge which isundesirable. Typically, the buried oxide (BOX) layer 90 comprises asilicon oxide material that has a thickness from about 200 to about 1000angstroms.

FIG. 4B shows the results of nitriding the buried oxide (BOX) layer 80to form a first nitrogen gradient 100 within a resulting buried oxide(BOX) layer 90′ near an interface of the semiconductor substrate 80 andthe resulting buried oxide (BOX) layer 90′. The first nitrogen gradient100 is formed using a thermal nitridation method. Other methods are notexcluded, provided that the other methods form a first nitrogen gradient(similar to the first nitrogen gradient 100) within the buried oxide(BOX) layer 90′ near an interface of the semiconductor substrate 80 andthe buried oxide (BOX) layer 90′.

The thermal nitridation method allows for diffusion of a nitridingmaterial through the buried oxide (BOX) layer 90 to the interfacethereof with the semiconductor substrate 80 when forming the buriedoxide (BOX) layer 90′. The thermal nitridation method uses a nitridingmaterial that may be selected from the group consisting of at leastnitrogen, nitrous oxide and nitric oxide nitriding materials. Typically,a thermal nitriding method in accordance with the instant embodimentuses a semiconductor substrate 80 and a buried oxide (BOX) layer 90temperature from about 500° to about 1350° degrees centigrade.Typically, the first nitrogen gradient 100 has a thickness from about 20to about 1000 angstroms within the buried oxide (BOX) layer 90′adjoining the interface thereof with the semiconductor substrate 80. Asdisclosed above, the first nitrogen gradient 100 has a nitrogen contentfrom about 0.1 to about 10 atomic percent within the buried oxide (BOX)layer 90′ (i.e., silicon:oxygen:nitrogen atomic ratio within the firstnitrogen gradient 100 is from about 333:666:1 to about 30:60:10).

FIG. 4C shows the results of further nitriding of the buried oxide (BOX)layer 90′ to form a second nitrogen gradient 110 at the surface of aburied oxide (BOX) layer 90″. The further nitriding that provides thesecond nitrogen gradient 110 within the buried oxide (BOX) layer 90″ iseffected using a plasma nitriding method that is effectively a surfacenitriding method and not a diffusional nitriding method. The plasmanitriding method may use the same nitriding materials as the thermalnitriding method disclosed above for forming the first nitrogen gradient100 within the buried oxide (BOX) layer 90′. The plasma nitriding methodmay use generally conventional plasma nitriding conditions, which may beselectively varied within conventional limitations, provided that thedimensions and the concentrations within the second nitrogen gradientare in accordance with the limitations described below.

Similarly with the first nitrogen gradient 100, the second nitrogengradient 110 typically also has a depth within the buried oxide (BOX)layer 90″ from about 20 to about 1000 angstroms. The second nitrogengradient 110 typically also has a silicon:oxygen:nitrogen atomic ratioin a range analogous to the range of the same parameter for the firstnitrogen gradient 100.

FIG. 4D shows the results of implanting hydrogen ions (with an implantdose from about 5e14 to about 5e16 hydrogen ions per square centimeter)into the semiconductor substrate 80 to form a fracture plane 120 withinthe semiconductor substrate 80. The fracture plane 120 is intended to belocated so as to leave a thickness from about 50 to about 2000 angstromsof the semiconductor substrate 80 to the first nitrogen gradient 100within the buried oxide (BOX) layer 90″. The energy of the hydrogenimplant is adjusted based on the position of the fracture plane 120.Alternatively to using the fracture plane 120, other methods foreventually cleaving the semiconductor substrate 80, or alternativelysimply thinning the semiconductor substrate 80, may also be used.

FIG. 4E shows a second semiconductor substrate 130 located upon andlaminated to the buried oxide (BOX) layer 90″. The second semiconductorsubstrate 130 may comprise a semiconductor material selected from thesame group of semiconductor materials as the semiconductor substrate 80.The semiconductor substrate 80 and the second semiconductor substrate130 may comprise the same semiconductor material or a differentsemiconductor material may be used. In addition, the semiconductorsubstrate 80 and the second semiconductor substrate 130 may have thesame or different dopant polarities, dopant concentrations andcrystallographic orientations. Typically, the second semiconductorsubstrate 130 comprises a silicon or silicon-germanium alloysemiconductor material that need not necessarily have the same dopantpolarity, dopant concentration and crystallographic orientation as thesemiconductor substrate 80.

FIG. 4F first shows the results of inverting the semiconductor structureof FIG. 4E.

FIG. 4F also shows the results of cleaving the semiconductor substrate80 along the fracture plane 120 illustrated in FIG. 4D and FIG. 4E toform therefrom a surface semiconductor layer 140 within asemiconductor-on-insulator (SOI) structure. Thesemiconductor-on-insulator (SOI) structure comprises the secondsemiconductor substrate 130 that serves as a base semiconductorsubstrate. The buried dielectric layer 90″ is located upon the secondsemiconductor substrate 130 wherein the second nitrogen gradient 110adjoins the second semiconductor substrate 130. The surfacesemiconductor layer 140 is located upon the buried oxide layer 90″,wherein the first nitrogen gradient 100 adjoins the surfacesemiconductor layer 140.

Cleavage of the semiconductor substrate 80 along the fracture plane 120that is illustrated in FIG. 4D and FIG. 4E to provide the surfacesemiconductor layer 140 may be effected using any of several methodsthat are conventional in the semiconductor fabrication art. Thermallyassisted cleavage methods and mechanically assisted cleavage methods arecommon. Mechanically assisted cleavage methods are typical. As is notedabove, alternative thinning methods to cleavage methods may also be usedfor forming the surface semiconductor layer 140 from the semiconductorsubstrate 80. Such alternative thinning methods may include, but are notlimited to chemical etch thinning methods, mechanical polish thinningmethods and chemical mechanical polish thinning methods.

FIG. 4F shows a semiconductor-on-insulator (SOI) structure fabricated inaccordance with an embodiment of the invention that comprises a firstembodiment of the invention. The semiconductor-on-insulator (SOI)structure comprises a second semiconductor substrate 130 that comprisesa base semiconductor substrate. A buried oxide (BOX) layer 90″ islaminated and located upon the second semiconductor substrate 130. Asurface semiconductor layer 140 is laminated and located upon the buriedoxide (BOX) layer 90″. The buried oxide (BOX) layer 90″ comprises asecond nitrogen gradient 110 located within a portion of the buriedoxide (BOX) layer 90″ that adjoins the second semiconductor substrate130. The buried oxide (BOX) layer 90″ also comprises a first nitrogengradient 100 located within a portion of the buried oxide (BOX) layer90″ that adjoins the surface semiconductor layer 140. The first nitrogengradient 100 and the second nitrogen gradient 110 inhibit mobile dopant(i.e., typically boron) migration from either the surface semiconductorlayer 140 or the second semiconductor substrate 130 and segregationthereof into the buried oxide (BOX) layer 90″.

The semiconductor-on-insulator (SOI) structure in accordance with thefirst embodiment as illustrated in FIG. 4F has abrupt and sharplydefined interfaces of the buried oxide (BOX) layer 90″ with the secondsemiconductor substrate 130 and the surface semiconductor layer 140. Theabrupt and sharply defined interfaces result from the use of adeposition method, a laminating method or a layer transfer method whenfabricating the semiconductor-on-insulator (SOI) structure. Either ofthe foregoing two abrupt and sharply defined interfaces of the buriedoxide (BOX) layer 90″ with the second semiconductor substrate 130 andthe surface semiconductor layer 140 typically has: (1) an interfacialroughness of less than about 10 angstroms RMS (where RMS is root meansquare, and intended to be calculated as a square root of a sum ofsquares of individual deviations from a mean value of a location of theinterface of the buried oxide layer 90″ with either of the surfacesemiconductor layer 140 or the second semiconductor substrate 130); and(2) a transition from the buried oxide (BOX) layer 90″ to either thesecond semiconductor substrate 130 or surface semiconductor layer 140within less than about five atomic layers. Atomic layer transitions maybe readily discerned using a cross-sectional image taken using atransmission electron microscope (TEM).

For comparison purposes, a semiconductor-on-insulator (SOI) structurethat is formed using a separation by implantation of oxygen (SIMOX)method is unlikely to achieve the foregoing interfacial roughnesslimitations or atomic layer transition limitations. When using aseparation by implantation of oxygen (SIMOX) method for forming asemiconductor-on-insulator (SOI) structure related to thesemiconductor-on-insulator (SOI) structure that is illustrated in FIG.4F, an interfacial roughness between a buried oxide (BOX) layer and asurface semiconductor layer or a base semiconductor substrate willtypically be greater than 10 angstroms RMS (i.e., root mean square).Similarly, transition from the buried oxide (BOX) layer to the surfacesemiconductor layer or the base semiconductor substrate for asemiconductor-on-insulator (SOI) structure fabricated using a separationby implantation of oxygen (SIMOX) method typically requires greater than5 atomic layers. The oxide/Si interface in a SIMOX wafer also typicallyhas higher fixed charge than the oxide/Si interface in an SOI substrateformed by lamination or wafer bonding.

FIG. 5A and FIG. 5B show a pair of schematic cross-sectional diagramsillustrating the results of progressive stages in fabricating asemiconductor-on-insulator (SOI) structure in accordance with anotherembodiment of the invention. This other embodiment of the inventioncomprises a second embodiment of the invention.

FIG. 5A shows a schematic cross-sectional diagram of asemiconductor-on-insulator (SOI) structure generally related to thefirst embodiment. The semiconductor-on-insulator (SOI) structure inaccordance with the second embodiment results from further processing ofthe semiconductor-on-insulator (SOI) structure of FIG. 4C within thefirst embodiment.

The semiconductor-on-insulator (SOI) structure of the second embodimentthat is illustrated in FIG. 5A comprises, in a first instance, thesemiconductor substrate 80 that comprises a base semiconductorsubstrate. The buried oxide (BOX) layer 90″ further comprising the firstnitrogen gradient 100 and the second nitrogen gradient 110 is locatedupon the semiconductor substrate 80. The first nitrogen gradient 100within the buried oxide (BOX) layer 90″ contacts the semiconductorsubstrate 80. The foregoing layers and structures may comprisematerials, have dimensions and be formed using methods analogous,equivalent or identical to the materials, dimensions and methods usedfor forming the like designated layers and structures that areillustrated in FIG. 4C.

The second embodiment also comprises: (1) a second buried oxide (BOX)layer 145 located upon the buried oxide layer 90″ and contacting thesecond nitrogen gradient 110; and (2) a second semiconductor substrate160 located upon the second buried oxide layer 145. The secondsemiconductor substrate 160 further comprises a fracture plane 150 thatresults from hydrogen ion implantation. Thus, within the secondembodiment, the buried oxide layer 90″ has an abrupt interface with thesecond buried oxide layer 145 rather than a surface semiconductor layerderived from the second semiconductor substrate 160 subsequent tocleavage of the second semiconductor substrate 160 at the fracture plane150.

The second buried oxide (BOX) layer 145 may comprise materials and beformed using methods analogous, equivalent or identical to the materialsand methods used for forming the buried oxide layer 80 that isillustrated in FIG. 4A (i.e., prior to nitridation thereof to form theburied oxide (BOX) layer 90′ or 90″). Thus, the second buried oxide(BOX) layer 145 will typically comprise an oxide comprising siliconand/or germanium, but absent nitrogen. Oxides of other elements are notexcluded. Typically, the second buried oxide (BOX) layer 145 has athickness from about 5 to about 100 angstroms.

The second semiconductor substrate 160 comprising the fracture plane 150is otherwise analogous to the semiconductor substrate 80 comprising thefracture plane 120 that is illustrated within FIG. 4E. The secondsemiconductor substrate 160 comprising the fracture plane 150 may beformed using methods analogous, equivalent or identical to the methodsused for forming the semiconductor substrate 120 comprising the fractureplane 80 within the first embodiment.

FIG. 5B shows the results of cleaving the semiconductor substrate 160along the fracture plane 150 that is illustrated in FIG. 5A to form asurface semiconductor layer 170 within a semiconductor-on-insulator(SOI) structure in accordance with the second embodiment of theinvention.

The semiconductor-on-insulator (SOI) structure in accordance with thesecond embodiment thus comprises a semiconductor substrate 80 thatcomprises a base semiconductor substrate. A buried oxide (BOX) layer 90″is located upon the semiconductor substrate 80. The buried oxide (BOX)layer 90'′ comprises a first nitrogen gradient 100 including a regionthat contacts the semiconductor substrate 80 and a second nitrogengradient 110 at an opposite region thereof. A second buried oxide (BOX)layer 145 contacts the second nitrogen gradient 110 within the buriedoxide (BOX) layer 90″. A surface semiconductor layer 170 contacts thesecond buried oxide (BOX) layer 145.

Within the second embodiment, the first nitrogen gradient 100 and thesecond nitrogen gradient 110 provide the same mobile dopant (i.e.,typically boron) migration and segregation inhibition that is discussedabove within the context of the first embodiment.

FIG. 6 shows a field effect transistor T′ located within the surfacesemiconductor layer 140 of the semiconductor-on-insulator (SOI)structure of FIG. 4F. The field effect transistor T' comprises a gatedielectric 200 located upon the surface semiconductor layer 140. A gateelectrode 210 is located upon the gate dielectric 200. A plurality ofspacer layers 220 is located adjoining sidewalls of the gate 210 andgate dielectric 200. A pair of source/drain regions 230 is locatedwithin the surface semiconductor layer 140 and separated by a channelregion located beneath the gate electrode 210. Extension region portions250 of the source/drain regions 230 are located beneath the spacerlayers 220.

Each of the foregoing layers and structures that comprise the fieldeffect transistor T′ may comprise materials, have dimensions and beformed using methods that are generally conventional in thesemiconductor fabrication art.

The gate dielectric 200 may comprise a generally conventional gatedielectric material such as an oxide, nitride or oxynitride of siliconhaving a dielectric constant from about 4 to about 20, measured invacuum. Oxides, nitrides and oxynitrides of other elements that are inthe same dielectric constant range are not excluded. The gate dielectric200 may also comprise a generally higher dielectric constant gatedielectric material having a dielectric constant from about 20 to atleast about 100, also measured in vacuum. Examples of such generallyhigher dielectric constant gate dielectric materials include, but arenot limited to: hafnium oxides, hafnium silicates, tantalum oxides,titanium oxides, lanthanum oxides, barium-strontium titanates (BSTs) andlead-zirconate titanates (PZTs).

The foregoing gate dielectric materials may be deposited using any ofseveral methods that are appropriate to their materials of composition,and are otherwise conventional in the semiconductor fabrication art.Non-limiting examples of methods include chemical vapor depositionmethods (including atomic layer chemical vapor deposition methods) andphysical vapor deposition methods (including sputtering methods).

The gate electrode 210 may similarly comprise any of several gateelectrode conductor materials. Non-limiting examples include certainmetals, metal alloys, metal silicides and metal nitrides. Also includedare doped polysilicon (i.e., having a dopant concentration from about 1e18 to about 1e22 dopant atoms per cubic centimeter) gate electrodematerials and polycide (doped polysilicon/metal silicide stack) gateelectrode materials.

The gate electrode conductor materials may also be formed using any ofseveral methods that are appropriate to their materials of composition.Non-limiting examples include plating methods, chemical vapor depositionmethods and physical vapor deposition methods.

The spacers 220 typically comprise a dielectric spacer material,although spacers comprising conductor spacer materials are also known inthe semiconductor fabrication art. The spacers 220 may alternativelycomprise a laminate or composite of dielectric materials and/orconductor materials. The spacers 220 are typically formed using ablanket layer deposition and anisotropic etch back method that providesthe spacers 220 with the characteristically inward pointed shape.

The source/drain regions 230 are typically formed using a two step ionimplantation method that uses the gate electrode 210, alternatively withand without the spacers 220, as a mask. A first step within the two stepion implantation method uses the gate electrode 210 absent the spacers220 as a mask and forms extension region portions 250 of thesource/drain regions 230 into the surface semiconductor layer 140. Asecond step within the two step ion implantation method uses the gateelectrode 210 and the spacers 220 as a mask to form conductor regionportions of the source/drain regions 230 into the surface semiconductorlayer 140.

As is illustrated within FIG. 6, the second semiconductor substrate 130and the extension region portions 250 of the source/drain region 230 maycomprise mobile dopants 40 (i.e., typically boron mobile dopants). Themobile dopants 40 are immobilized with respect to migration andsegregation into the buried oxide (BOX) layer 90″ due to the presence ofthe first nitrogen gradient 100 and the second nitrogen gradient 110.

FIG. 6 shows a schematic cross-sectional diagram of a field effecttransistor structure T′ located within a semiconductor-on-insulator(SOI) structure in accordance with the first embodiment of the inventionas illustrated in FIG. 4F. The field effect transistor structure T′comprises a quantity of mobile dopants 40 within both: (1) the secondsemiconductor substrate 130 that comprises a base semiconductorsubstrate; and (2) the surface semiconductor layer 140. The mobiledopants 40 migration and segregation into the buried oxide (BOX) layer90″ is impeded by the presence of the first nitrogen gradient 110 withinthe buried oxide (BOX) layer 90″ adjoining the surface semiconductorlayer 140 and the second nitrogen gradient 100 within the buried oxide(BOX) layer 90″ adjoining the second semiconductor substrate 130.

FIG. 7 shows a schematic cross-sectional diagram of a field effecttransistor T′ located within a semiconductor-on-insulator (SOI)structure in accordance with the second embodiment of the invention asillustrated in FIG. 5B.

FIG. 7 is largely analogous to FIG. 6, but differs with respect to: (1)presence of the semiconductor substrate 80 as a base semiconductorsubstrate rather than the second semiconductor substrate 130 as the basesemiconductor substrate; (2) an inversion of the buried oxide (BOX)layer 90″; (3) incorporation of the second buried oxide (BOX) layer 145;and (4) presence of the surface semiconductor layer 170 in place of thesurface semiconductor layer 140.

Within the schematic diagram of FIG. 7, the mobile dopants 40 within thetransistor T′ are still immobilized by the second nitrogen gradient 110from migration into the buried oxide (BOX) layer 90″, but mobile dopants40 may migrate and segregate into the second buried oxide (BOX) layer145 that does not include a nitrogen gradient. The semiconductorstructure that is illustrated in FIG. 7 results in significantly higherFET drive current (and hence improved circuit performance) due toreduced device parasitic resistance in the extension and source/drainregions.

Described and illustrated within the context of the foregoingembodiments are structures and methods for minimizing mobile dopant(i.e., boron) migration and segregation from a surface semiconductorlayer and a base semiconductor substrate in to a buried oxide (BOX)layer within a semiconductor-on-insulator (SOI) structure. Inhibition ofsuch mobile dopant migration and segregation enables the fabrication oflow extension sheet resistance pMOSFETs within comparatively thinsurface semiconductor layers (i.e., less than about 350 angstroms)within semiconductor-on-insulator (SOI) structures. Thus, higherperformance devices, such as in particular pMOSFET devices, are enabledby the foregoing embodiments. Preventing mobile dopant migration andsegregation into a buried oxide (BOX) layer within asemiconductor-on-insulator (SOI) structure is also beneficial forfully-depleted semiconductor-on-insulator (SOI) based devices where asurface semiconductor layer doping is used as a back gate and used toset a threshold voltage.

It should be noted that although the drawings and description providedabove show nitrogen gradients at the interface between both of the basesemiconductor layer and the surface semiconductor layer, the presentinvention is not so limited. For example, the nitrogen gradient can belocated at only the interface between the base semiconductor layer orthe interface between the surface semiconductor layer.

The preferred embodiments of the invention are illustrative of theinvention rather than limiting of the invention. Revisions andmodifications may be made to methods, materials structures anddimensions of a semiconductor structure in accordance with the preferredembodiments of the invention while still providing a semiconductorstructure in accordance with the invention, further in accordance withthe accompanying claims.

1. A method for fabricating a semiconductor-on-insulator structurecomprising: nitriding, while using a thermal nitriding method, an oxidelayer located upon a base semiconductor substrate to provide a nitridedoxide layer having a first nitrogen gradient that peaks at an interfaceof the nitrided oxide layer with the base semiconductor substrate, saidthermal nitriding method uses a nitriding material selected from thegroup consisting of a nitrogen nitriding material, a nitric oxidenitriding material and a nitrous oxide nitriding material and a thermaldiffusion temperature from about 500 to about 1350 degrees centigrade;nitriding, while using a plasma nitriding method, the nitrided oxidelayer to provide a twice nitrided oxide layer having a second nitrogengradient that peaks at the exposed surface of the twice nitrided oxidelayer, said plasma nitriding method uses a nitriding material selectedfrom the group consisting of a nitrogen nitriding material, a nitricoxide nitriding material and a nitrous oxide nitriding material; andlaminating a surface semiconductor layer upon the twice nitrided oxidelayer to contact the second nitrogen gradient, wherein first nitrogengradient and the second nitrogen gradient has a depth from about 20 toabout 1000 angstroms within the twice nitrided oxide layer.